The invention generally relates to measuring temperatures of workpieces to be treated by glow discharge for carburizing, decarburizing, metalizing, hardening or the like and, more particularly, to measuring temperatures of workpieces to be nitrided in the presence of a nitrogeneous atmosphere, commonly ammonia gas.
For example, the nitrogen case-hardening process which is termed "nitriding" consists in subjecting machined and preferably heat-treated parts to the action of a nitrogeneous medium, commonly partially dissociated ammonia gas, under certain conditions whereby surface hardness is imparted to the material without necessitating any further treatment. The nitriding process imparts to the metal to be case-hardened vastly improved properties such as increased wear resistance, retention of hardness at elevated temperatures, and resistance to certain types of corrosion.
It has been proposed to nitride a workpiece by electrically connecting a container in which a workpiece is accommodated to a positive terminal of an electrical current source, and by electrically connecting the workpiece itself to a negative terminal of the electrical current source. The source is of sufficient strength to establish an electrical potential or voltage gradient between the container and the workpiece so as to produce about the exterior surface of the workpiece a glow discharge field which accelerates the ions in the nitrogeneous atmosphere surrounding the workpiece to thereby heat the workpiece and harden the exterior surface of the latter. A glow discharge is a discharge brought about as a result of ionization of a gas medium surrounding a conductor.
It has been proposed to measure the temperature to which the workpiece is heated by inserting a thermal sensor element, such as a thermocouple, in a bore either already available or deliberately machined in the workpiece. However, the wires, which conduct the electrical signal generated from the temperature detected at the heated workpiece in direction towards the exterior of the container to an indicating instrument, intersect and pass through the glow discharge field. This results in the decided disadvantage that the ionized field undesirably influences and interferes with the electrical signal carried by the wires and that atoms can be embedded in the thermocouple and wire material which influence the signals, too. The measurement data is no longer accurate or reliable. In extrem cases of ion concentration the sensor element can be fully destroyed.
Furthermore it is only possible to measure at a relatively large distance relative to the exterior surface of the workpiece. However, this necessitates a rather long waiting time until the heat at the exterior surface of the workpiece reaches the deeply embedded sensor element. This is evidently a very inefficient technique.